The present invention relates to a meter overdrive protection circuit, and more particularly, to a programmable meter drive circuit having an overdrive protection scheme.
In developing a speedometer or tachometer for a highway vehicle such as a truck, it is desirable to utilize a programmable pulse source as a meter drive. Differences in tire size and gear ratios for the particular truck models can be programmed into the pulse source such that a particular width pulse is utilized for a particular truck. The width of the pulse is related to revolutions of the transmission shaft in the case of a speedometer or revolutions of the drive shaft in the case of a tachometer. A microcomputer is usually utilized to program the pulse generator to produce pulses of a width necessary to provide proper meter deflection. The programmable pulse generator usually comprises an integrated circuit chip having a pulse generation mode. The output pulses from the pulse generator are utilized to drive the meter. The meter drive typically comprises an electromechanical meter movement which performs a time averaging of the pulses received from the pulse generator and produces a corresponding deflection in the meter needle.
The pulse generator is dependent on a trigger signal received from and generated by a reluctance sensor coil positioned with a rotating gear on the appropriate rotating shaft of the vehicle. In a tachometer application, such gear is usually the fly-wheel, and in a speedometer application, such shaft is usually the transmission output drive shaft. The rotating device contains teeth which induce a current in the reluctance coil sensor. Such current is of a sinusoidal nature, and this current is input to the pulse generator to typically cause the triggering of the pulse generator upon receipt of a positive waveform from the sensor.
In the event of an overspeed condition of the rotating device, an excessive number of trigger signals will be received by the pulse generator. The width of the pulse generator output waves will not change, but rather the frequency of such output waves will increase due to the increased frequeny of trigger signals. Accordingly, the meter deflection will increase and correctly reflect the increased rate of rotation of the shaft being measured. However, if the frequency of such triggering signals exceeds a certain rate, the time average value of the pulse generator output would exceed the preselected value required for full scale meter deflection and cause the needle to be pinned against a mechanical stop. It is an object of the present invention to provide a scheme of avoiding the mechanical pinning and possible damage to the meter needle.
Another problem which could occur in the event of an overspeed operation of the rotating device is that the triggering signals from the sensor device could increase in frequency to a point such that, due to the width of the pulse generator output pulses, a triggering signal could be received during a pulse generator on condition. Such a triggering signal would be ignored since the pulse generator was already in an on condition, and accordingly an incorrect reading of the rotation rate of the shaft would be reflected in the meter device. Accordingly, it is another object of the present invention to provide a method of avoiding such incorrect meter indication of an overspeed condition.